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Internet Engineering Task Force Mark Allman
INTERNET DRAFT NASA Lewis/Sterling Software
File: draft-ietf-tcpsat-stand-mech-00.txt Dan Glover
NASA Lewis
October 2, 1997
Expires: April 2, 1998
Enhancing TCP Over Satellite Channels
using Standard Mechanisms
Status of this Memo
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six
months and may be updated, replaced, or obsoleted by other documents
at any time. It is inappropriate to use Internet-Drafts as
reference material or to cite them other than as ``work in
progress.''
To learn the current status of any Internet-Draft, please check the
``1id-abstracts.txt'' listing contained in the Internet- Drafts
Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).
NOTE
This draft is not to be taken as a complete work. In the future
this draft will be extended (or another draft written) to discuss
TCP mechanisms that are not yet on the IETF standards track. These
mechanisms either need further research or have been determined to
be inappropriate for general purpose use on a shared network, but
may be beneficial for private satellite networks.
Abstract
The Transmission Control Protocol (TCP) provides reliable delivery
of data across any network path, including network paths containing
satellite channels. While TCP works over satellite channels there
are several mechanisms that enable TCP to more effectively utilize
the available capacity of the network path. This draft outlines
some of these TCP mitigations. At this time, all mitigations
discussed in this draft are IETF standards track mechanisms.
1. Introduction
Satellite channel characteristics have an effect on the way
transport protocols, such as the Transmission Control Protocol (TCP)
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[Pos81], behave. When protocols such as TCP perform poorly, channel
utilization is low. This draft is divided up as follows: Section 2
provides a brief outline of the characteristics of satellite
networks. Section 3 outlines two non-TCP mechanisms that enable TCP
to more effectively utilize the available bandwidth. Section 4
outlines the TCP mechanisms defined by the IETF that benefit
satellite networks. Finally, Section 5 provides a summary of what
modern TCP implementations should include to be considered
"satellite friendly".
2. Satellite Characteristics
There is an inherent delay in the delivery of a message over a
satellite link due to the finite speed of light and the altitude of
communications satellites.
Many communications satellites are located at Geostationary Orbit
(GSO) with an altitude of approximately 36,000 km [Sta94]. At this
altitude the orbit period is the same as the Earth's rotation
period. Therefore, each ground station is always able to "see" the
orbiting satellite. The propagation time for a radio signal to
travel twice that distance (corresponding to a ground station
directly below the satellite) is 239.6 milliseconds (ms) [Mar78].
For ground stations at the edge of the view area of the satellite,
the distance traveled is 2 x 41,756 km for a total propagation delay
of 279.0 ms [Mar78]. These delays are for one ground
station-to-satellite-to-ground station route (or "hop"). Therefore,
the propagation delay for a message and its reply (one round-trip
time or RTT) would be no more than 558 ms. The delay will be
proportionately longer if the link includes multiple hops or if
intersatellite links are used. As satellites become more complex
and include on-board processing of signals, additional delay may be
added.
Other orbits are possible for use by communications satellites
including Low Earth Orbit (LEO) and Medium Earth Orbit (MEO)
[Mar78]. The lower orbits require the use of constellations of
satellites for constant coverage. In other words, as one satellite
leaves the ground station's sight, another satellite appears on the
horizon and the channel is switched to it. The propagation delay to
a LEO orbit ranges from several milliseconds when communicating with
a satellite directly overhead, to as much as 80 ms when the
satellite is on the horizon. These systems are more likely to use
intersatellite links and have variable path delay depending on
routing through the network.
Satellite channels are dominated by two fundamental characteristics,
as described below:
NOISE - The strength of a radio signal falls in proportion to
the square of the distance traveled. For a satellite link the
distance is large and so the signal becomes weak before reaching
it's destination. This results in a low signal-to-noise ratio.
Typical bit error rates for a satellite link today are on the
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order of 10-7. Satellite error performance equivalent to fiber
will become common as advanced error control coding is used in
new systems. However, many current satellite systems do not
provide error free service.
BANDWIDTH - The radio spectrum is a limited natural resource,
hence the bandwidth available to satellite systems is limited.
Typical carrier frequencies for current, point-to-point,
commercial, satellite services are 6 GHz (uplink) and 4 GHz
(downlink), also known as C band, and 14/12 GHz (Ku band). A
new service at 30/20 GHz (Ka band) will be emerging over the
next few years. Traditional C band transponder bandwidth is
typically 36 MHz to accommodate one color television channel (or
1200 voice channels). Ku band transponders are typically around
50 MHz. Furthermore, one satellite may carry a few dozen
transponders.
Not only is bandwidth limited by nature, but the allocations for
commercial communications are limited by international agreements so
that this scarce resource can be used fairly by many different
applications.
Although satellites have certain disadvantages when compared to
fiber channels, they also have certain advantages over terrestrial
links. First, satellites have a natural broadcast capability.
Next, satellites can reach geographically remote areas or countries
that have little terrestrial infrastructure.
Satellite channels have several characteristics that differ from
most terrestrial channels. These characteristics can degrade the
performance of TCP. These characteristics include:
Long feedback loop
Due to the propagation delay of some satellite channels (e.g.,
approximately 250 ms over a geosynchronous satellite) it takes a
large amount of time for a TCP sender to determine whether or
not a packet has been successfully received at the final
destination. This delay hurts interactive applications such as
telnet, as well as some of the TCP congestion control algorithms
(see section 4).
Large delay*bandwidth product
The delay*bandwidth product (DBP) defines the amount of data a
protocol should have "in flight" (data that has been
transmitted, but not yet acknowledged) at any one time to fully
utilize the available channel capacity. Because the delay in
some satellite environments is large, TCP will need to keep a
large amount of data "in flight".
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Transmission errors
Some satellite channels exhibit a higher bit-error rate (BER)
than typical terrestrial networks. TCP uses all packet drops as
signals of congestion and reduces the sending rate. Packets
dropped due to corruption do not indicate that the network is
congested. However, the TCP sender cannot determine why a
packet was dropped and therefore will reduce the transmission
rate for all packet drops.
Asymmetric use
Due to the expense of the equipment used to send data to
satellites, asymmetric satellite networks are often constructed.
For example, a host connected to such a network will send all
outgoing traffic over a slow terrestrial link (such as a dialup
modem channel) and receive incoming traffic via the satellite
channel. Another common situation arises when both the incoming
and outgoing traffic are sent using a satellite link, but the
uplink has less available capacity than the downlink. This
asymmetry can have a large impact on TCP performance.
Variable Round Trip Times
In some satellite environments, such as low-Earth orbit (LEO)
constellations, the propagation delay to and from the satellite
varies over time. This can have a negative impact on TCP's
ability to accurately set retransmission timeouts and determine
the appropriate window size.
Intermittent connectivity
In satellite orbit configurations, TCP connections must be
transfered from one satellite to another or from one ground
station to another from time to time. This handoff can cause
packet loss.
Most satellite channels only exhibit a subset of the above
characteristics. In addition, some terrestrial networks exhibit
some of the above characteristics, as well. The mechanisms
outlined in this document should benefits most networks,
especially those with one of the above characteristics.
3. Lower Level Mitigations
It is recommended that those utilizing satellite channels in their
networks should use the following two non-TCP mechanisms which can
increase TCP performance. These mechanisms are Path MTU Discovery
and forward error correction (FEC) and are outlined in the following
two sections.
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3.1 Path MTU Discovery
Path MTU discovery [MD90] is used to determine the maximum packet
size a connection can use on a given network path without being
subjected to IP packet fragmentation. The sender transmits a packet
that is the appropriate size for the local network with which it is
connected (e.g., 1500 bytes on an Ethernet) and sets the IP "don't
fragment" (DF) bit. If the packet is too large for a channel along
the network path, the gateway that would normally fragment the
packet and forward the fragments will return an ICMP message to the
originator of the packet. The ICMP message will indicate that the
original segment could not be transmitted without being fragmented
and will also contain the maximum size that can be forwarded by the
gateway.
This allows TCP to use the largest possible packet size, without
incurring the cost of fragmentation and reassembly. Large packets
reduce the packet overhead by sending more data bytes per overhead
byte. Also, larger packets allow the slow start and congestion
avoidance algorithms to increase the congestion window more rapidly
(as outlined in section 4).
The disadvantage of Path MTU Discovery is that it may cause a long
pause before TCP is able to start sending data. For example, assume
a packet is sent with the DF bit set and one of the intervening
gateway (G1) returns an ICMP message indicating that it cannot
forward the segment. At this point, the sending host reduces the
packet size to the size returned by G1 and sends another packet with
the DF bit set. The packet will be forwarded by G1, however this
does not ensure all subsequent gateways in the network path will be
able to forward the segment. If a second gateway (G2) can not
forward the segment it will return an ICMP message to the
transmitting host and the process will be repeated. Therefore, path
MTU discovery can waste a large amount of time determining the
maximum allowable packet size on the network path and the network
topology. However, in practice, Path MTU Discovery is not that
expensive.
3.2 Forward Error Correction
A loss event in TCP is always interpreted as an indication of
congestion and always causes TCP to reduce the window size. When
loss occurs during slow start, then slow start is terminated and TCP
enters congestion avoidance. Premature termination of slow start
and entry into congestion avoidance due to losses other than
congestion losses will cause needless inefficiency in channel
utilization. Furthermore, drops due to corruption causes TCP to
needlessly reduce the amount of data being injected into the
network.
For TCP to operate efficiently, the channel characteristics should be
such that nearly all loss is due to network congestion. The use of
forward error correction coding (FEC) on a satellite link should be
used to bring the performance of the link to at least fiber quality.
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Because of the effect of long RTT, errors on a satellite link have
more severe repercussions than on a lower RTT terrestrial channel
[PS97]. There are some applications, such as military jamming,
where FEC cannot be expected to solve the noise problem.
4. Standard TCP Mechanisms
This section includes an outline of the mechanisms that may be
necessary in satellite or hybrid satellite/terrestrial networks to
better utilize the available capacity of the link. These mechanisms
may also be needed to fully utilize fast terrestrial channels.
Furthermore, these mechanisms do not fundamentally hurt performance
in a shared terrestrial network. Each of the following sections
outlines one mechanism and why that mechanism may be needed.
4.1 Congestion Control
To avoid generating an inappropriate amount of network traffic for
the current network conditions TCP employs four congestion control
mechanisms [JK88] [Jac90] [Ste97]. These algorithms are slow start,
congestion avoidance, fast retransmit and fast recovery. These
algorithms are used to adjust the amount of unacknowledged data that
can be injected into the network and to retransmit segments dropped
by the network.
TCP uses two variables to accomplish congestion control. The first
variable is the congestion window (cwnd). This is an upper bound on
the amount of data the sender can inject into the network before
receiving an acknowledgment (ACK). The value of cwnd is limited to
the receiver's advertised window. The congestion window is
increased or decreased during the transfer based on the inferred
amount of congestion present in the network. The second variable is
the slow start threshold (ssthresh). This variable determines which
algorithm is being used to increase the value of cwnd. If cwnd is
less than ssthresh the slow start algorithm is used to increase the
value of cwnd. However, if cwnd is greater than or equal to
ssthresh the congestion avoidance algorithm is used. The initial
value of ssthresh is the receiver's advertised window size.
Furthermore, the value of ssthresh is reduced when congestion is
detected.
The four congestion control algorithms are outlined below, followed
by a brief discussion of the impact of satellite environments on
these algorithms.
4.1.1 Slow Start and Congestion Avoidance
When a host begins sending data on a TCP connection the host has no
knowledge of the current state of the network between itself and the
data receiver. In order to avoid transmitting an inappropriately
large burst of traffic, the data sender is required to use the slow
start algorithm at the beginning of a transfer [JK88] [Bra89]
[Ste97]. Slow start begins by initializing cwnd to 1 segment. This
forces TCP to transmit one segment and wait for the corresponding
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ACK. For each ACK that is received, the value of cwnd is increased
by 1 segment. For example, after the first ACK is received cwnd
will be 2 segments and the sender will be allowed to transmit 2 data
packets. This continues until cwnd meets or exceeds ssthresh, or
loss is detected.
When the value of cwnd is greater than or equal to ssthresh the
congestion avoidance algorithm is used to increase cwnd [JK88]
[Bra89] [Ste97]. This algorithm increases the size of cwnd more
slowly than does slow start. Congestion avoidance is used to probe
the network for any additional capacity. During congestion
avoidance, cwnd is increased by 1/cwnd for each incoming ACK.
Therefore, if one ACK is received for every data segment, cwnd will
increase by 1 segment per round-trip time (RTT).
Long-delay satellite networks force poor utilization of the
available channel bandwidth when using the slow start and congestion
control algorithms [All97]. For example, transmission begins with
the transmission of one segment. After the first segment is
transmitted the data sender is forced to wait for the corresponding
ACK. When using a GSO satellite this leads to an idle time of
roughly 500 ms when no useful work is being accomplished.
Therefore, slow start takes more real time over GSO satellites than
on typical terrestrial channels. This holds for congestion
avoidance, as well [All97]. This is precisely why Path MTU Discovery
is an important algorithm. While the number of segments we transmit
is determined by the congestion control algorithms, the size of
these segments is not. Therefore, using larger packets will enable
TCP to send more data per segment which yields better channel
utilization.
4.1.2 Fast Retransmit and Fast Recovery
TCP's default mechanism to detect dropped segments is a timeout
[Pos81]. In other words, if the sender does not receive an ACK for
a given packet within the expected amount of time the segment will
be retransmitted. The retransmission timeout (RTO) is based on
observations of the RTT. In addition to retransmitting a segment
when the RTO expires, TCP also uses the lost segment as an
indication of congestion in the network. In response to the
congestion, the value of ssthresh is set to half of the cwnd and the
value of cwnd is then reduced to 1 segment. This triggers the use
of the slow start algorithm to increase cwnd until the value of cwnd
reaches half of its value when congestion was detected. After the
slow start phase, the congestion avoidance algorithm is used to
probe the network for additional capacity.
TCP ACKs always acknowledge the highest in-order segment that has
arrived. Therefore an ACK for segment X also effectively ACKs all
segments < X. Furthermore, if a segment arrives out-of-order the
ACK triggered will be for the highest in-order segment, rather than
the segment that just arrived. For example, assume segment 11 has
been dropped somewhere in the network and segment 12 arrives at the
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receiver. The receiver is going to send a duplicate ACK covering
segment 10 (and all previous segments).
The fast retransmit algorithm uses these duplicate ACKs to detect
lost segments. If 3 duplicate ACKs arrive at the data originator,
TCP assumes that a segment has been lost and retransmits the missing
segment without waiting for the RTO to expire. After a segment is
resent using fast retransmit, the fast recovery algorithm is used to
adjust the congestion window. First, the value of ssthresh is set
to half of the value of cwnd. Next, the value of cwnd is halved.
Finally, the value of cwnd is artificially increased by 1 segment
for each duplicate ACK that has arrived. The artificial inflation
can be done because each duplicate ACK represents 1 segment that has
left the network. When the cwnd permits, TCP is able to transmit
new data. This allows TCP to keep data flowing through the network
at half the rate it was when loss was detected. When an ACK for the
retransmitted packet arrives, the value of cwnd is reduced back to
ssthresh (half the value of cwnd when the congestion was detected).
Fast retransmit can resend only one segment per window of data sent.
When multiple segments are lost in a given window of data, one of
the segments will be resent using fast retransmit and the rest of
the dropped segments must wait for the RTO to expire, which causes
TCP to revert to slow start.
TCP's response to congestion differs based on the way the congestion
was detected. If the retransmission timer causes a packet to be
resent, TCP drops ssthresh to half the current cwnd and reduces the
value of cwnd to 1 segment (thus triggering slow start). However,
if a segment is resent via fast retransmit both ssthresh and cwnd
are set to half the current value of cwnd and congestion avoidance
is used to send new data. The difference is that when
retransmitting due to duplicate ACKs, TCP knows that packets are
still flowing through the network and can therefore infer that the
congestion is not that bad. However, when resending a packet due to
a the expiration of the retransmission timer, TCP cannot infer
anything about the state of the network and therefore must proceed
conservatively by sending new data using the slow start algorithm.
4.1.3 Congestion Control in Satellite Environment
The above algorithms have a negative impact on the performance of
individual TCP connection's performance, especially over long-delay
satellite channels [All97] [AHKO97]. However, the algorithms are
necessary to prevent congestive collapse in a shared network [JK88].
Therefore, the negative impact on a given connection is more than
offset by the benefit to the entire network.
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4.2 Large TCP Windows
The standard TCP window size (65,535 bytes) is not adequate to allow
a single TCP connection to utilize the entire bandwidth available on
some satellite channels. TCP throughput is limited by the following
formula [Pos81]:
throughput = window size / RTT
Therefore, using the maximum window size of 65,535 bytes and a
geosynchronous satellite channel RTT of 560 ms [Kru95] the maximum
throughput is limited to:
throughput = 65,535 bytes / 560 ms = 117,027 bytes/second
Therefore, a single standard TCP connection cannot fully utilize,
for example, T1 rate (192,000 bytes/second) GSO satellite channels.
However, TCP has been extended to support larger windows [JBB92].
The window scaling options outlined in [JBB92] should be used in
satellite environments, as well as the companion algorithms PAWS
(Protection Against Wrapped Sequence space) and RTTM (Round-Trip
Time Measurements).
4.3 Selective Acknowledgments
Selective acknowledgments (SACKs) [MMFR96] allow TCP receivers to
inform TCP senders exactly which packets have arrived. TCP senders
that do not use SACKs must infer which segments have not arrived and
retransmit accordingly. This can lead to needless retransmissions,
in the case when the sender infers incorrectly. When utilizing
SACKs, the sender does not need to guess which segments have not
arrived.
Some satellite channels require the use of large TCP windows to
fully utilize the available capacity, as discussed above. With the
use of large windows, the likelihood of losing multiple segments in
a given window of data increases. When multiple segments are lost,
SACKs will ensure the data sender retransmits only those segments
that were dropped and not those that safely arrived at the
receiver.
5. Mitigation Summary
Table 1 summarizes the mechanisms that have been discussed in this
document. Those mechanisms denoted "Recommended" are IETF standards
track mechanisms that are recommended by the authors for use in
networks containing satellite channels. Those mechanisms marked
"Required" have been defined by the IETF as required for hosts using
the shared Internet [Bra89].
Satellite users should check with their TCP vendors (implementors)
to ensure the recommended mechanisms are supported in their stack in
current and/or future versions.
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Work on improving the efficiency of TCP over satellite channels is
ongoing and will be summarized in a planned memo along with other
considerations, such as network architectures.
Mechanism Use Section
+------------------------+-------------+------------+
| Path-MTU Discovery | Recommended | 3.1 |
| FEC | Recommended | 3.2 |
| TCP Congestion Control | | |
| Slow Start | Required | 4.1.1 |
| Congestion Avoidance | Required | 4.1.1 |
| Fast Retransmit | Recommended | 4.1.2 |
| Fast Recovery | Recommended | 4.1.2 |
| TCP Large Windows | | |
| Window Scaling | Recommended | 4.2 |
| PAWS | Recommended | 4.2 |
| RTTM | Recommended | 4.2 |
| TCP SACKs | Recommended | 4.3 |
+------------------------+-------------+------------+
Table 1
6. Security
The recommendations contained in this memo do not alter the security
implications of TCP.
References
[AHKO97] Mark Allman, Chris Hayes, Hans Kruse, and Shawn Ostermann.
TCP Performance Over Satellite Links. In Proceedings of the 5th
International Conference on Telecommunication Systems, March
1997.
[All97] Mark Allman. Improving TCP Performance Over Satellite
Channels. Master's thesis, Ohio University, June 1997.
[Bra89] Robert Braden. Requirements for Internet Hosts --
Communication Layers, October 1989. RFC 1122.
[Jac90] Van Jacobson. Modified TCP Congestion Avoidance Algorithm.
Technical Report, LBL, April 1990.
[JBB92] Van Jacobson, Robert Braden, and David Borman. TCP
Extensions for High Performance, May 1992. RFC 1323.
[JK88] Van Jacobson and Michael Karels. Congestion Avoidance and
Control. In ACM SIGCOMM, 1988.
[Mar78] James Martin. Communications Satellite Systems. Prentice
Hall, 1978.
[MD90] Jeff Mogul and Steve Deering. Path MTU Discovery, November
1990. RFC 1191.
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[MMFR96] Matt Mathis, Jamshid Mahdavi, Sally Floyd, and Allyn
Romanow. TCP Selective Acknowledgment Options, October 1996.
RFC 2018.
[Pos81] Jon Postel. Transmission Control Protocol, September 1981.
RFC 793.
[PS97] Craig Partridge and Tim Shepard. TCP Performance Over
Satellite Links. IEEE Network, 11(5), September/October 1997.
[Sta94] William Stallings. Data and Computer Communications.
MacMillian, 4th edition, 1994.
[Ste97] W. Richard Stevens. TCP Slow Start, Congestion Avoidance,
Fast Retransmit, and Fast Recovery Algorithms, January 1997.
RFC 2001.
Author's Addresses:
Mark Allman
NASA Lewis Research Center/Sterling Software
21000 Brookpark Rd. MS 54-2
Cleveland, OH 44135
mallman@lerc.nasa.gov
http://gigahertz.lerc.nasa.gov/~mallman
Dan Glover
NASA Lewis Research Center
21000 Brookpark Rd. MS 54-2
Cleveland, OH 44135
Daniel.R.Glover@lerc.nasa.gov
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